Riddle at Wayne Manor

What used to be gold, these days is green;
If you have too much, count by machine.
Now it comes in plastic, and moves quickly by wire;
Have plenty of this in order to retire.

The sweaty figure frowned, squinting in the sunlight.

"Good morning, Master Bruce. I see you are just returning from your morning run. Are you feeling better today, sir?"

The figure looked at him, admiring his consistently cheerful demeanor and persistently optimistic outlook.

"Good morning, Alfred. Much better, thanks."

"Have you found any assistance?" he asked, the inquisitive tone in his voice aluding to their conversation of the previous evening.

"Yes, as a matter of fact, I have. Thanks for asking." The sweaty figure wiped his face. "With help from the right place, it's not so hard to be The Batman." He paused, then added, "Many are the days when I think you could do this."

"Many are the days when I think I already am doing it, sir."

They both smiled.

The paper in the sweaty figure's hand caught Alfred's eye. Knowing he sensed its significance, but was too discrete to ask about it, the sweaty figure handed it to him.

"I found this at the front gate this morning. What do you make of it, Alfred?"

"What used to be gold, these days is green; If you have too much, count by machine. Now it comes in plastic, and moves quickly by wire; Have plenty of this in order to retire." Alfred looked up. "It's a riddle, sir."

"The answer is money. In the old days, people used coins made of precious metals. Kings and governments fixed the values of the coins, but the coins themselves had intrinsic worth, independent of that assigned to them." He paused. "Now, people use paper money; possibly the most important in the world's market is the US dollar, which is green. When in large quantities, money is counted by machines."

"Of course," Alfred continued. "Today there are credit and debit cards, electronic funds transfers...."

"Banking, investing... money moves around the globe electronically. And, people are concerned about whether they will have enough for retirement."

"But who would send a riddle to Bruce Wayne?"

"That, Alfred, is the riddle."